JPH05161079A - Imaging device having nonvolatile frame memory function - Google Patents

Abstract

PURPOSE:To obtain the imaging device having a nonvolatile frame memory in which a storage time is almost infinite and its amplification factor and S/N reach a practical level by providing a nonvolatile memory in the vicinity of a gate section of a short channel FET. CONSTITUTION:A potential of a photoelectromotive force from a photodiode array 21 is applied to the series connection between a nonvolatile frame memory 20 and a gate-source parasitic capacitance CSD of a short-channel FET 10 and when a voltage division to the memory 20 exceeds a threshold voltage, polarization is written in the memory 20 as information and kept. Then a charge is stored in the capacitor CSD corresponding to the polarization caused in the memory 20 to keep a prescribed potential to the upper electrode. The potential is a potential of a gate 16 of the FET 10, the FET 10 is closed and a current flows to a read amplifier 27, from which the current is delivered out. Through the constitution above, the imaging device with a frame memory function is obtained, in which the storage time is almost infinite, the amplification factor and the S/N reach a practical level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup device for a video capturing device, and more particularly to an image pickup device having a frame memory function.

[0002]

2. Description of the Related Art Generally, there are various image pickup devices which capture an image represented by a CCD and output it as an electric signal.

Among these image pickup devices, there is "Television Technical Report".
(vol11, No28 P43; 1987. Mizoguchi et al.), high sensitivity SIT as shown in FIGS. 7 (a) and 7 (b).
There is an image sensor. In this high-sensitivity SIT image pickup device, an amplification element 1 is provided for each cell, and the gate 1a of the amplification element 1 is provided.
The photodiode 2 is connected to. Figure 7
As shown in the sectional view of (c), a vertical junction FET is used as the amplifying element.

In the example of the image pickup device shown in FIGS. 7A and 7C, a pn junction type photodiode 2 and a MOS type photodiode 3 are used as photodiodes, respectively. Each diode part has each gate 1
Although connected to a and 3a, these gates serve as capacitors and have a charge storage function. However,
The life of charge storage is only about 10 sec due to electron-hole recombination in each diode section.

In addition, as a non-volatile memory element, IEEE JOURANAL OF SOLD STATECIRCUITS (VOL, .23,
No. 5, OCTOBER 1988. JTEVAN et al.) Discloses a ferroelectric memory in which a MOS gate FET as shown in FIG. 8 is used as an image pickup element and a ferroelectric capacitor is used as a charge storage section. This element constitutes a non-volatile memory by utilizing the hysteresis characteristic of a ferroelectric substance. By turning on a MOSFET 4 provided for each memory cell, a desired memory is selected and its output is sense amplifier 5.
It is designed to be read by.

[0006]

However, the above-mentioned FIG.
In the image pickup device shown in (1), the image pickup device using the short-channel FET has a charge storage function, but since the time for holding the charge is as short as about 10 seconds,
Especially when used as a frame memory, there is a drawback that the storage time for holding information is too short.

Further, since the memory element shown in FIG. 8 uses a ferroelectric memory, it is non-volatile and has little problem in terms of storage time.
Since the S-type switch is used, the amplification factor and the S / N ratio are low, and it has not reached a practical level.

It is therefore an object of the present invention to provide an image pickup device having a nonvolatile frame memory function, which has a semi-infinite storage time and has an amplification factor and an S / N ratio which have reached practical levels.

[0009]

In order to achieve the above object, the present invention is provided for each of a plurality of arranged pixel cells,
Photoelectric conversion means for photoelectrically converting incident light, which includes either a photodiode using pn coupling or a photodiode using a biased MOS diode, selection means for selecting a desired pixel cell, and the photoelectric conversion means. One end is connected to the output terminal of the non-volatile memory, which is composed of a ferroelectric capacitor for storing the information signal from the photoelectric conversion means, and the control electrode is connected to the other end of the non-volatile memory. Provided is an imaging device having a nonvolatile frame memory function, which is composed of a short channel FET composed of either a charge fluctuation device (CMD) or a static induction transistor (SIT), which amplifies and outputs a read information signal. To do.

[0010]

In the image pickup device having the non-volatile frame memory function configured as described above, polarization is induced in the ferroelectric capacitor corresponding to the photoelectromotive force, the polarization of the capacitor is held, and the gate of the short channel FET is As the polarization continues to be induced in the capacitor, a current flows in the read amplifier.

[0011]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1A shows the structure of the image sensor as the first embodiment, and FIG. 1B shows the equivalent circuit thereof.

In the structure of this image pickup device, first, a short channel (MOS) FET is formed on a semiconductor substrate. This short channel FET 10 is n ⁺ The n-type epitaxial layer 12 is formed on the shaped substrate 11, the source 13 and the drain 14 are provided, and the gate 16 is provided via the gate insulating film 15.

A ferroelectric capacitor 20 including a lower electrode 17, a ferroelectric 18 and an upper electrode 19 is formed near the short channel FET 10. The short channel F
The gate 16 of the ET 10 is connected to the lower electrode 17.

Further, n photodiodes are stacked to form a photodiode array 21, a cathode 22 at one end is grounded, and an anode 23 at the other end is the upper electrode 1.
9 is connected.

In the equivalent circuit of FIG. 1B, the photodiode array 21 and the ferroelectric capacitor 20 are connected in series as described above, and the lower electrode 17 of the ferroelectric capacitor 20 is connected to the gate 16 of the short channel FET 10. Are connected. A gate-source parasitic capacitance C _SD exists between the gate 16 and the ground. In the short channel FET 10, the source 13 is connected to the other terminal of the read amplifier 27 whose one terminal of the input part is grounded and the bit line Vy, and the drain 14 is connected to the drive line Vx.

In the photodiode array 21, since n photodiodes are stacked, the electromotive force Vph of one photodiode is nVph with respect to the threshold voltage V _th1 of one photodiode shown in FIG. > V _th1 (1)

At the time, ferroelectric polarization is induced. The photodiode array 21 is shown in FIG.
As shown in (c), an n-type epitaxial layer 25 is formed on an n + type substrate 24, and a PNP layer 26 is formed in the n-type epitaxial layer 25. The operation of the image sensor thus configured will be described.

One of the gate-source parasitic capacitance C _SD of the short channel FET is connected in series with the ferroelectric capacitor and the other is at ground potential. One of the input terminals of the read amplifier 27 is also at ground potential.

A potential due to photovoltaic is applied to the series connection of the gate-source parasitic capacitance C _SD and the ferroelectric capacitor 20, and the partial voltage of the ferroelectric capacitor 20 becomes a threshold voltage V.
_{When th1} is exceeded, the polarization (size and direction) is written and held in the ferroelectric capacitor 20 as information.

[0020] In response to the polarization generated in the ferroelectric capacitor 20, also causes charges to be accumulated in the capacitor C _SD between the gate and the source, the potential of the upper electrode of the capacitor C _SD between the gate and source of constant value The potential is also the gate potential of the short-channel FET, and the short-channel FET is turned on and a current flows, which is output from the reading amplifier 27.

Next, FIG. 2 shows the structure of an image pickup device as a second embodiment. Here, in the constituent member of the second embodiment,
The same components as those in the first embodiment are designated by the same reference numerals and the description thereof will be omitted.

The second embodiment is a modification of the first embodiment shown in FIG. 1, in which a switch FET 28 for controlling writing to a ferroelectric capacitor is provided with an anode 23 of a photodiode array 21 and a ferroelectric material. Upper electrode 1 of capacitor 10
It will be replenished between 9 and. The photodiode for improving the sensitivity and the dynamic range of the image sensor includes a battery 29 or a bias charge storage capacitor 29 for boosting the ground potential to the bias potential V _bias.
'Is provided. In this case, when V _ph + V _bias > V _th1 (2), ferroelectric polarization is induced. Next, in FIG.
The configuration of an image sensor as an example will be shown and described. Figure 3
FIG. 3A shows a sectional structure of the third embodiment, and FIG.
Shows the equivalent circuit.

In this structure, first, n + type substrate 30 is provided with n
The n-type epitaxial layer 31 is formed, and the p-type photodiode 32 and the n-type photodiode 33 are formed in the n-type epitaxial layer 31.

The P-type photodiode 32 has the n-type structure.
After the p-type diffusion layer 34 is formed in the p-type epitaxial layer 31 by thermal diffusion or the like, ions are implanted into a predetermined region of the p-type diffusion layer 34 to form the source 35 and the drain 36 of the n-type layer. A gate 37 is formed between the source 35 and the drain 36 via an insulating film. Further, the N-type photodiode 33 is ion-implanted into a predetermined region in the n-type epitaxial layer 31 to form a p-type source 38 and a drain 39. A gate 40 is formed between the source 38 and the drain 39 via an insulating film. Further, a ferroelectric capacitor 44 including a lower electrode 41, a ferroelectric 42 and an upper electrode 43 is formed in the vicinity of these.

A battery 4 for applying a bias to the gate 37 of the P-type photodiode 32 and the upper electrode 43.
5 is provided, and a predetermined bias is applied to the gate 37.
Further, a battery 46 for bias application is provided on the gate 40 of the N-type photodiode 33 and the lower electrode 41, and a predetermined bias is applied to the lower electrode 41. In the third embodiment, the depletion layer 4 under the MOS gate is used as a photoelectric conversion unit.
This is an example of using 7, 48 as a MOS photodiode. The operation of the third embodiment will be described with reference to the equivalent circuit of FIG.

In this image pickup device, each channel portion of the P-type photodiode 32 and the N-type photodiode 33 functions as a MOS photodiode. In addition, a capacitor C _1a ,
There are C _1b , C _2a , and C _2b (the size of each capacitance is C _CS ).

When light is incident from the outside, the MOS photodiode 47 further injects negative charges into the capacitors C _1a and C _2a , and the potential applied to the ferroelectric capacitor 44 increases via the capacitor C _1a . Similarly, positive charges are injected into the capacitors C _1b and C _2b , and the potential applied to the ferroelectric capacitor 44 via the capacitor C _1b increases. By increasing the potential applied to the ferroelectric capacitor 44, the threshold voltage exceeds V _th1 , the polarization is written in the ferroelectric capacitor 44, and the applied voltage is maintained (maintained).

Therefore, the capacitors C _1a , C _1b ,
The charges of C _2a and C _2b are also retained. By holding these charges, the source / drain currents of the P and N type short channel FETs change, and the changes are output and read.

Further, in FIG. 3A, the batteries 45 and 46 are used as the members for applying the bias, but in FIG.
As a member for applying the bias, a capacitor 50,
An application example using 51 is shown.

Next, FIG. 5 shows an example of the configuration of an image pickup device as a fourth embodiment, which will be described. Here, in the constituent members of the fourth embodiment, constituent members equivalent to those in the first embodiment are designated by the same reference numerals.

In this structure, the gate 16 of the short channel FET 10 is connected to the anode 23 of the photodiode array 21.
Also, the upper electrode 19 of the ferroelectric capacitor 20 is connected. Further, the cathode 22 and the lower electrode 17 are grounded to each other. Furthermore, the gate 16 of the short channel FET 10
A gate-source capacitance 52 whose other end is grounded is connected to. The source 13 of the short-channel FET 10 is connected to the other end of the read amplifier 52 whose one input end is grounded, and the read information is output.

As described in detail above, the image pickup device of the present invention is
The image sensor has a continuous frame memory function, which increases the amount of stored charge and thus increases the dynamic range.

The present invention is not limited to the above-described embodiment, but various modifications and changes can be made. For example, by changing the write bias applied to the ferroelectric memory 20, it is possible to provide the ferroelectric memory with a plurality of polarization states as shown in FIG.
It is possible to provide an image sensor capable of multi-value recording and having a nonvolatile frame memory function. Of course, various modifications and applications are possible without departing from the spirit of the invention.

[0034]

As described above, according to the present invention, there is provided an image pickup device having a non-volatile frame memory function, which has a semi-infinite storage time, and has an amplification factor and an S / N ratio which have reached practical levels. be able to.

[Brief description of drawings]

FIG. 1 (a) is a diagram showing a structure of an image pickup device of a first embodiment according to the present invention, FIG. 1 (b) is a diagram showing an equivalent circuit thereof, and FIG. 1 (c) is a photo. It is a figure which shows the concrete structure of a diode row.

FIG. 2 is a diagram showing a configuration of an image sensor according to a second embodiment of the present invention.

FIG. 3 (a) is a diagram showing a sectional structure of a third embodiment according to the present invention, and FIG. 3 (b) is a diagram showing an equivalent circuit thereof.

FIG. 4 is a diagram showing an application example in which a capacitor is used instead of the battery of the bias applying member of FIG. 3 (a).

FIG. 5 is a diagram showing a configuration of an image sensor according to a fourth embodiment of the present invention.

FIG. 6 is a diagram showing a hysteresis characteristic curve of a ferroelectric capacitor having multi-valued polarization.

7A is a diagram showing a configuration of a conventional high-sensitivity SIT image pickup device, FIG. 7B is a sectional view showing the structure, and FIG. 7C is a vertical junction as an amplifying device. FET
It is a figure which shows the cross section of.

FIG. 8 is a short channel F of a conventional MOS gate.
It is a figure which shows the structure of the image pick-up element which used ET as an amplification element.

[Explanation of symbols]

1 ... Amplifying element, 1a, 3a ... Gate, 2 ... Pn junction type photodiode, 3 ... MOS type photodiode, 4 ...
MOSFET, 5 ... Sense amplifier, 10 ... Short channel FET, 11 ... N ⁺ Substrate, 12 ... N-type epitaxial layer, 13, 35, 38 ... Source, 14, 36, 39 ... Drain, 15 ... Gate insulating film, 16, 37, 40 ... Gate, 17, 41 ... Lower electrode, 18, 42 … Ferroelectric, 1
9, 43 ... Upper electrode, 20, 44 ... Ferroelectric capacitor, 21 ... Photodiode array, 22 ... Cathode, 23
... Anode, 24 ... N + type substrate, 25 ... N type epitaxial layer, 26 ... PNP layer, 27, 52 ... Readout amplifier, 28 ... Switch FET, 29, 45, 46 ... Battery,
29 '... Bias charge storage capacitor, 30 ... n + type substrate, 31 ... N type epitaxial layer, 32 ... P type photodiode, 33 ... N type photodiode, 34 ... P type diffusion layer, 47, 48 ... MOS photo diode _{(depletion), 50,51, C 1a, C} 1b, C 2a, C 2b ... capacitor, Vy ... bit lines, Vx ... drive line, C _SD ... gate-source parasitic capacitance, V _th1 ... photodiode threshold Voltage.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication H01L 27/146

Claims (3)

[Claims] 1. A photoelectric conversion unit that is provided for each of a plurality of arranged pixel cells and photoelectrically converts incident light, a selection unit that selects a desired pixel cell, and one end of which is connected to an output terminal of the photoelectric conversion unit. A non-volatile memory for accumulating an information signal from the photoelectric conversion means, and a short-channel FET for amplifying and outputting an information signal read from the non-volatile memory by connecting a control electrode to the other end of the non-volatile memory. An image pickup device having a non-volatile frame memory function, comprising: 2. The short channel FET is composed of either a charge fluctuation device (CMD) or a static induction transistor (SIT), and the non-volatile memory is composed of a ferroelectric capacitor memory. 1. An image pickup device having a nonvolatile frame memory function described in 1. 3. The photoelectric conversion means comprises one of a photodiode using a pn coupling and a photodiode using a biased MOS diode, and the photodiodes are connected in series in a plurality of stages. An image pickup device having a non-volatile frame memory function according to claim 1.